1. Field of the Invention
The present invention relates to an ignition apparatus including a spark plug, an ignition coil and a coil driver of an internal combustion engine (hereinafter also referred to as an “engine”), and more particularly, it relates to a new advanced technology for a coil driver for driving the ignition coil.
2. Description of the Related Art
In general, in an ignition apparatus for an internal combustion engine that is provided with an ignition coil having a primary coil and a secondary coil and a switching element (e.g., IGBT) for turning on and off (energize and deenergize) a primary current of the ignition coil, a predetermined high voltage (ignition voltage) generated at the secondary coil side when the primary current to the primary coil is switched from an energized state (on) into an interrupted state (off) is impressed to a spark plug, whereby a spark occurs at the spark plug, thereby driving the engine.
However, on the contrary, when the primary current is switched from the interrupted state (off) into the energized state (on), a turn-on voltage (opposite in polarity to the ignition voltage) generated at the secondary coil side is impressed to the spark plug to generate a spark therefrom at a timing other than the predetermined timing, whereby abnormality vibration or the like is caused to the engine. Accordingly, there is a possibility that normal operation of the engine is disturbed or inhibited.
Accordingly, there has also been proposed an ignition apparatus for an internal combustion engine in which the value of the turn-on voltage generated upon energization of the primary current is made lower than the voltage value of a normal ignition voltage that causes a spark plug to generate a spark upon interruption of the primary current (see, for example, a first patent document: Japanese patent application laid-open No. H10-176647).
In the conventional motor control apparatus described in the above-mentioned first patent document, it is constructed such that the turn-on voltage generated when the energization of the primary coil is switched from the off state to the on state is not impressed to the spark plug by inserting a high voltage diode having a withstand voltage (e.g., about 5 [kV]) higher than the turn-on voltage in a low-voltage side terminal (or a high-voltage side terminal) of the secondary coil with the turn-on voltage acting as a reverse voltage.
In this case, the high voltage diode is in general an expensive and large-sized discrete part, and is constructed as a circuit separated from the coil driver having the switching element. A high voltage generated upon interruption of the primary current is in a forward direction with respect to the high voltage diode, so a voltage impressed to the spark plug is not made lower and an energy loss never occurs.
In the conventional ignition apparatus for an internal combustion engine, the high voltage diode in the form of the expensive and large-sized discrete part is used so as to prevent the turn-on voltage from being impressed to the spark plug. As a result, there is a problem that an increase in the number of man-hours and an increase in cost are invited when the high voltage diode is built into the ignition coil.
In addition, there is also a problem that the degree of freedom in mounting the high voltage diode is limited because of the high voltage diode being in the form of the discrete part.
Further, when the high voltage diode for prevention of the turn-on voltage is to be mounted on a substrate of the coil driver, there will exist a low voltage system (e.g., an IC including a current limiting circuit and a waveform shaping circuit) having a low withstand voltage against a high voltage on the substrate of the coil driver in a mixed relation with the high voltage diode, so in order to separate the high voltage diode and the low voltage system in the coil driver from each other, it is necessary to arrange these parts in a divided manner by the use of a plurality of substrates or to form a guard or the like for protection of the low voltage system. Accordingly, there is a further problem that the number of production steps or processes and the number of component parts increase, thus inviting an increase in cost.